Laboratory Methods for Assessing Platelet Function
PFA-100 (Platelet Function Analyzer-100)
The PFA-100 is NOT recommended for diagnosing inherited platelet function disorders due to insufficient specificity and sensitivity 1.
How It Works
The PFA-100 measures platelet-related primary hemostasis by simulating high shear stress conditions 2, 3:
- Whole blood flows through a capillary tube under high shear stress
- Blood encounters a membrane coated with platelet agonists (collagen plus either epinephrine or ADP)
- Platelets adhere and aggregate at the aperture, eventually occluding it
- Results are reported as closure time (CT) - the time required to occlude the aperture
- Two cartridge types: CEPI (collagen/epinephrine) and CADP (collagen/ADP)
Clinical Performance
- CEPI cartridges detect aspirin effects with 71.7% sensitivity and 97.8% positive predictive value 3
- CADP cartridges detect qualitative platelet defects but NOT aspirin use 3
- Reference intervals: mean CT of 132 seconds for CEPI and 93 seconds for CADP 3
- Cannot reliably detect clopidogrel effect - only 29% of clopidogrel-treated patients showed prolonged closure times 4
Key Limitations
Despite 94.9% sensitivity and 88.8% specificity in some studies 3, the ISTH guidelines explicitly state this test lacks adequate diagnostic performance for inherited platelet disorders 1. It may have utility in monitoring antiplatelet therapy, particularly aspirin, but should not be used as a primary diagnostic tool for platelet dysfunction.
Platelet Aggregation (Light Transmission Aggregometry - LTA)
Light transmission aggregometry is the gold standard for evaluating platelet function and diagnosing inherited platelet function disorders 1.
How It Works
LTA measures platelet aggregation in platelet-rich plasma (PRP) 2:
- Blood is anticoagulated with citrate and centrifuged to obtain PRP
- PRP is placed in a cuvette with continuous stirring at 37°C
- Platelet agonists are added to stimulate aggregation
- As platelets aggregate, light transmission through the sample increases
- Results are recorded as percentage aggregation over time
Agonist Panel
First-step testing uses standard agonists 1:
- ADP (multiple concentrations)
- Collagen
- Ristocetin (to detect von Willebrand disease and platelet-type VWD)
- Arachidonic acid (to detect aspirin effect and COX-1 deficiency)
- Epinephrine (though response may be absent in healthy individuals)
Second-step testing with expanded agonist panel 1:
- α-thrombin
- TRAP-6
- U46619 (thromboxane A2 mimetic)
- CRP and convulxin (collagen receptor agonists)
- PAR-4 activating peptide
- PMA and A23287 (calcium ionophore)
Clinical Utility
- Detects up to 40% of inherited platelet function disorders with first-step testing 1
- Sensitivity of 94.3% and specificity of 88.3% for platelet defects 3
- 100% sensitivity for detecting aspirin effect 3
- More sensitive than PFA-100 for detecting clopidogrel effect (71% vs 34% inhibition detected) 4
Important Caveats
- Requires 21-28 mL of blood for comprehensive first-step testing 1
- Whole blood impedance aggregometry may be more sensitive than optical aggregometry for detecting clopidogrel effects, as red blood cells modulate platelet function 4
- Epinephrine responses can be variable even in healthy individuals 1
- Must be performed within hours of blood collection for optimal results
Diagnostic Algorithm
When platelet dysfunction is suspected, begin with LTA using the standard agonist panel along with flow cytometry for glycoprotein expression 1. If initial testing is non-diagnostic but clinical suspicion remains high, proceed to expanded agonist panel testing combined with additional specialized tests (flow cytometry with expanded antibodies, transmission electron microscopy, granule content measurement) 1.